Major lipid components in cell membrane include glycerolipid, sphingolipid, and cholesterol. Glycerolipid in animal cells has a structure in which 2 molecules of a fatty acid and a phosphate group are bound to a glycerol skeleton, and has, upon dispersion in an aqueous solution, a two layered membrane structure in which a hydrophobic fatty acid residue is present inside and a phosphate group portion is present outside. The formed two layered membrane structure has a property of liquid crystalline, and in 1972 Singer and Nicolson (United States) proposed a “fluid mosaic model” in which a protein is floating in such liquid crystal.
In addition to the cell membrane for separating the cell from outside, the cell also has a complicated membrane structure inside and has organelles having their own respective characteristic functions. A portion which constitutes a cell membrane is synthesized by an organelle referred to as an “endoplasmic reticulum” and is finally transported to a cell membrane through a Golgi apparatus.
Glycerolipid which is one component of a cell membrane, contains a fatty acid and a phosphate group. However, the types of fatty acid are various, and the phosphate group portion also has variety, for example, it may contain choline, ethanolamine, serine, or inositol. Further, sphingolipid to which a variety of saccharides such as glucose, galactose, or lactose are bound, exists. There are several tens of thousands of lipid molecules in the natural world if all the above lipids are included. The composition of the lipid varies depending on the type of organisms, the type of organs, the type of cells, and the type of organelles. Furthermore, the lipid composition of the inner layer constituting the lipid bilayer of the biomembrane is different from that of the outer layer.
Unlike glycerolipid, since sphingolipid has a base referred to as “sphingosine” as a skeleton, it can be either donor or acceptor of a hydrogen bond. Also, in general, sphingolipid binds to a long-chained fatty acid. Due to this structural character of sphingolipid, in the case of a plurality of sphingolipids, the hydrophilic portions are likely to aggregate through hydrogen bonding while the hydrophobic portions are likely to aggregate through the hydrophobic interaction of fatty acid chains. Simons et al. (Germany) proposed that sphingolipids on the cell membrane aggregate and form a lipid domain. They named this a “lipid raft” (hereinafter merely referred to as a “raft”).
Ever since the concept of the “raft” as described above was proposed, it has been suggested that the raft plays an important role in the signal transduction, as well as cell adhesion, infection of viruses and bacteria, polymer translocation in the cell, and the like. In recent years, it has been shown that among sphingolipids, sphingomyelin is a necessary and enough factor in the formation of the raft.
The present inventors have heretofore found that Lysenin, which is a toxic protein secreted by Eisenia foetida, specifically binds to sphingomyelin.
Lysenin is already known and the existence of three types of Lysenin 1, 2, and 3 is known. Lysenin 1 is composed of 297 amino acid residues, and Lysenins 2 and 3 are composed of 300 amino acid residues. Among these, Lysenins 1 and 3 have high cytotoxicity and, thus, they could not be used for labeling sphingomyelin in living cells. Lysenin 2 does not recognize sphingomyelin. Therefore, Lysenins which have been heretofore reported, could not be put to practical use as sphingomyelin detecting probes.